Protective circuit for photoelectric tubes



I 50 gm June l5, 1954 PROTECTIVE CIRCUIT FOR PHOTOELECTRIC 'TUBES Filed July 2l, 1951 J. BALL. 2,681,417

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`is not Patented June 15, 1954 UNITED STATES PATENT OFFICE PROTECTIVE CIRCUIT FOR PHOTO- ELECTRIC TUBES Jack Ball, Chesterland, Ohio, assigner to Picker X-Ray Corporation, Waite Manufacturing 7 Claims. l

This invention relates to X-ray apparatus and particularly to equipment for protecting the photo-tube which is customarily used for automatically terminating the X-ray exposure in accordance with predetermined conditions of operation.

Heretofore, photo-tubes have been utilized as part of the equipment employed for automatically terminating an X-ray exposure, and such equipment has included a trigger tube and condenser in the output circuit of the photo-tube, and the electrical characteristics thereof have been such that the trigger tube would operate to disconnect the power supply to the input of the X-ray tube after the photo-tube has been operated for a sufficient length of time to charge the condenser. In practice, the photo-tube, together with its associated fluorescent screen, has been mounted in a box beneath the X-ray table, and, in such position, the screen has been protected against irnpingement of light thereon from any source other than the X-ray beam. Recent developments, however, in X-ray apparatus, particularly in the use of a spot lmer wherein the photo-tube is positioned above the X-ray table and is used in conjunction with a movable frame which carries the photo-sensitive lm, the photo-tube has, of necessity, been placed in a position wherLit l rotected a 'n in ement of light otler than from the X-ray tube. Thus, if light,

which' is used for general illumination inthe room wherein the X-ray exposure is being made, should be inadvertently turned on, or if someone should attempt to make an exposure, by way of demonstration, without darkening the room, then the photo-tube would be subjected to such an excessive amount of light as to damage the photo-tube before the condenser could be charged suiliciently to re the trigger tube.

A further problem in the operation of phototubes has been the fact that although the trigger tube might function normally to open the circuit to the X-ray tube, nevertheless, if the operator should inadvertently maintain pressure upon the push-button control member after the circuit to the X-ray tube has been disconnected, then a high voltage would still be impressed upon the phototube, with consequent liability to damage it in case the screen at such time should be subjected to an abnormal amount of light.

An object of the present invention is to provide an apparatus by means of which the photo-tube can be located in an exposed position and will still be protected against damage in the event of impingement of an excessive amount of light upon the photo-tube screen, irrespective of whether or not the X-ray tube is connected to its source of current supply, and irrespective of whether or not the operator manually maintains the pushbutton control member closed after the power supply to the X-ray tube has been disconnected. The invention contemplates a protective apparatus which will permit the trigger tube assembly, heretofore utilized, to function, upon the impingement of a normal amount of radiation upon the phototube screen during use of the X-ray equipment, to disconnect the power input thereto, in the event of the occurrence of an excessive amount of radiation impingement upon the screen.

The present invention has solved a problem in the protection of the photo-tube by disconnecting the power input thereto automatically whenever the output of the tube exceeds a predetermined amount. Under such conditions, the apparatus operates automatically to disconnect the power supply to the X-ray tube, as well as to the photo-tube, and prevents normal opera- Vtion of the equipment until the defect is remedied.

Brieiiy, the present invention includes an auxillary trigger tube with a suitable resistance connected in the output circuit of the photo-tube and intermediate the primary trigger tube assembly heretofore used, and the photo-tube. The characteristics of the resistance are such that the auxiliary trigger tube will not be fired, consequent upon the normal flow of current in the output of the photo-tube, whereby the desired selected interval of time for operation of the X-ray tube can be utilized for charging the condenser and iiring the primary trigger tube. 'I'he firing of that tube operates a relay to disconnect the power input supply to the X-ray tube, but if at any time the photo-tube is connected to its power supply and if at such time an excessive amount of radiation is impinged upon the tube screen, then the resistance used in the circuit of the auxilitry trigger tube will cause such tube to be fired which, in turn, will energize a relay to disconnect the input power to the X-ray tube and to the photo-tube respectively. Thus, even though the operator should maintain the manually operable switch closed so that the power supply to the photo-tube is maintained, nevertheless, the present invention will operate to protect the phototube against damage until the source of excess impingement of light upon the tube screen is eliminated.

The drawing is a schematic diagram showing a circuit arrangement for X-ray apparatus embodying the present invention.

In the drawing, I designates an X-ray tube, the power input to which is supplied from a source of alternating current through lines L-I and L-2 through a k. v. selector indicated in general at I3, which is connected to the primary winding I4 of a power transformer I5. The secondary I6 of the transformer is connected to the X-ray tube through a bridge rectifier indicated in general at I'I. The circuit for the primary winding of the power transformer includes a normally open switch 20.

The X-ray apparatus includes a photo-tube 25 with its associated fluorescent screen 2E. The tube may be of the Zworykin electron-multiplier type with its customary associated network of resistances 30, density control potentiometer 3|, and calibrator 32, these being adjustable for the type of lm that is to be used. Energizing current is supplied to the photo-tube at L--S and L-4 through a transformer 35, the secondary of which includes in its circuit a diode 35, the plate of which is connected to one end of the secondary winding of the transformer 35, and the cathode of which is connected to the potentiometer 3 I. A condenser 45 is shunted across the circuit of the secondary winding.

For operating the X-ray tube under normal conditions, there is provided a primary trigger circuit, to which current is supplied at the leads Such circuit includes a manually operable push button control switch 45, a relay 46, a, normally closed switch 4l, and a normally closed switch 48, and the arrangement is such that when the switch 45 is closed the relay d6 is energized and the switch 2G is thereby closed.

Thus the X-ray tube is connected to its source of power supply. Shunted across such circuit is a relay 50 which controls the operation of a primary trigger circuit by means of which the X-ray tube is controlled under conditions of the photo-tube charges the condenser sufficiently to re the tube 6I which in turn energizes a relay E3 from a source of current supply through the leads L-'I and L-8 to open the switch 41. This interrupts the ilow of current in the circuit )Lr-5 and L-t thus deenergizing the relay 4S and allowing the switch to open. This disconnects the source of supply to the X-ray tube.

Under normal conditions of operation, the primary trigger circuit operates to interrupt the source of current supply to the X-ray tube after the tube has been operated for an intended length of time, conditioned upon the intensity and duration of radiation desired. Such interval of time is co-extensive with the time required to charge the condenser C, and the charging of the condenser interposes a time lag of a desired extent, so that when the trigger tube 69 fires, a relay will be energized which would disconnect the X-ray tube from its source of power supply. The present invention embodies an auxiliary trigger' circuit which includes a trigger tube 'I5 and a resistor I6 connected in the output circuit of the photo-tube and adapted for energizing a relay to disconnect the power supply to the X-ray tube and also to the photo-tube. The electrical characteristics of the resistor 'I6 are such that under normal conditions of operation, the output from the photo-tube will charge the condenser C and re the primary trigger tube but, under abnormal conditions of operation, such as upon the impingement of radiation upon the screen in excess of that intended for normal operation, the output will fire the auxiliary trigger tube 15, thereby energizing the relay and interrupting the source of current supply to the X-ray tube and to the photo-tube, as aforesaid.

The leads L-'I and L-B provide a direct current supply at 16S volts for the trigger tubes 6I and I5 while the leads L-I and L-S provide a direct current supply at 100 volts for operation of the amplier tube Sii. The lead L-'I is shown as being connected to ground and to the resistance network for the photo-tube.

I have found, where the input to the phototube from the secondary to the transformer is of the order of 1,000 volts, that normal operation of the primary trigger tube unit will occur if the condenser C has the capacity of .1 mfd.. and if the resistors in the primary trigger tube circuit have the following values.

I have also foundthat the photo-tube will be adequately protected if the resistor 'I6 has a resistance of 500,000 ohms. By placing the resistor 'I0 between the condenser C and the photo-tube, in circuit with the anode 65 thereof, then the trigger tube 'I5 can be fired upon the occurrence of an electrical surge in the phototube output circuit even though the surge occurs within the space of time required for one cycle. Under normal operations, the photo-timer is usually operated within the range of IAS of a second up to 10 seconds, this being determined by the setting of the capacity of the condenser. and the calibrators 3| and 32, it being understood that the calibration is initially preset in accordance with the electrical characteristics of the photo-tube and of the sensitivity of the nlm, and in accordance with the k. v. setting of the power supply for the X-ray tube. These settings are well known in the art.

In operation, assuming that a source of 220 volts of alternating current supply is provided for the leads L-I and I r-Z, that a source of 110 volts of alternating current supply is provided for the leads L-3 and L4, that a source of 160 volts direct current supply is provided for the leads L-'I and L-B and a source of direct current supply of 100 volts is provided across the leads L-1 and L-9, and assuming that the X-ray tube and photo-tube, with its associated network and protective circuits, are connected as aforesaid, then whenever the operator closes the push-button switch 45, a circuit is established which energizes the relay 46 thereby closing the switch 20 and completing the circuit for the power input to the X-ray tube. At the same time the relay 50 is energized which opens the switch (which normally shunts out the condenser C), thereby putting the condenser into the output circuit of the photo-tube. Energization of the relay 50 also closes the switches 56 and 51. When the switch 56 is closed, then the primary trigger tube is cocked and the input circuit to the primary\winding of the photo-tube transformer 35 is established. Under normal operation, the output from the photo-tube will charge the condenser C which when adequately charged will fire the trigger tube 6l thus energizing the relay 63. Energization of the relay 63 opens the switch 41, thereby deenergizing the relay 4E,V opening the switch and disconnecting the power supply to the X-ray tube.

Under normal conditions of operation as aforesaid, the trigger tube 15 would not be iired. However, if an abnormal amount of radiation should strike the screen 26 so as to create a surge in the output from the photo tube, then irrespective of the amount of charge upon the condenser C, the auxiliary trigger tube will be fired thereby enabling the flow of current across the lines L-l and L-8 through relay 90 which, when ener. gized, opens the switches 48 and 92, the first of which opens the circuit through the relay 46 thereby interrupting the source of current supply to the X-ray tube, as aforesaid, While opening of the switch 92 interrupts the source of current supply to the photo-tube. This condition prevails even though the operator may maintain pressure manually against the push button control' 45. Thus, it -is necessary to remedy the abnormal condition which is causing the impingement of excessive radiation upon the screen 26 before normal conditions of operation can be reestablished.

Under normal conditions, a iiow of current of the order of micro-amperes, or less, in the output circuit of the photo-tube, will operate to iire the trigger tube 6|, whereas an abnormal flow of current, of the order of 100 microamperes will operate to cause a suilicient voltage drop across resistor 16 to re the trigger tube 15.

As soon, however, as the operator opens the switch 45, the relay 50 is deenergized, whereupon the switch 51 is automatically opened and the supply circuit to the photo-tube is automatically disconnected. Deenergization of the relay 50 also enables the switch 55 to be automatically closed, thereby shunting the condenser out of the circuit and enables the switch 56 to be automatically opened, which deenergizes the relay 63. This allows the switch 41 to close, thereby resetting the conditions for normal operation upon a succeeding actuating of the push button control member l5.

An advantage of the present invention is that the photo-tube is protected under abnormal conditions of operation, consequent upon the impingement of an abnormal amount of radiation upon the photo-tube screen. The apparatus embodying the present invention can be readily applied to existing X-ray apparatus without extensive changes in the construction thereof, and is equally adapted for inclusion in original equipment.

I claim:

l. In combination, an X-ray tube, an electrical circuit connecting said tube to a source of current supply, switch means in said circuit, a phototube having an anode, a light-sensitive cathode and multiplying electrodes, a fluorescent screen positioned adjacent said photo-tube, a trigger tube having an anode, a cathode and a control grid, an electrical circuit, including a capacitor sensitive to iiring'of said trigger tube for actuating said switch means to disconnect said X-ray tube from its source of current supply, consequent upon charging of the capacitor under normal conditions of impingement of radation upon said screen during operation of said X-ray tube,

a second trigger tube having an anode, a cathode and a control grid, a connection, including a resistor, between the anode of the photo-tube and the grid of the second trigger tube, switch means operatively connected with an electrical circuit sensitive to the iiring of said second trigger tube independently of the charge on said capacitor for terminating the operation of the photo-tube consequent upon the impingement of an abnormal amount of radiation upon said fluorescent screen during the X-ray exposure.

2. An X-ray exposure control circuit comprising in combination, a photo-tube having an anode, a light-sensitive cathode and multiplying electrodes, a source of current supply for said photo-tube, a Iiuorescent screen positioned adjacent said photo-tube, a trigger tube having an anode, a cathode and a control grid and adapted to iire at a predetermined potential, switch means coacting in response to an electrical circuit sensitive to the firing of said trigger tube for disconnecting the source of current supply to the phototube, and a circuit connecting the anode of the photo-tube and the control grid of the trigger tube, said circuit beingr connected to utilize the photo-tube current for developing the predetermined potential to iire the trigger tube before the impingement of radiation upon the iiuorescent screen can reach an amount suiiicient to cause damage to the photo-tube.

3. In combination, an X-ray tube, an electrical circuit connecting said tube to a source of current supply, switch means in said circuit, a photo-tube having an anode, a light sensitive cathode and multiplying electrodes, a fluorescent screen positioned adjacent said photo-tube, a trigger tube having an anode, a cathode and a control grid, an electrical circuit sensitive to iii-ing of said trigger tube for actuating said switch means to terminate said X-ray exposure consequent upon operation of said X-ray tube under` normal conditions for a predetermined length of time, a second trigger tube having an anode, a cathode and a control grid, an electrical circuit coupling the second trigger tube to the photo-tube and including means for utilizing the photo-tube current to iire said second trigger tube consequent upon the impingement of an abnormal amount of radiation upon said iiuorescent screen during the X-ray exposure, and an electro-responsive switch having energizing means in said coupling circuit responsive to liring of said second trigger tube for positioning the switch to terminate the operation of the photo-tube.

4. In combination, an X-ray tube, an electrical circuit connecting said tube to a source of current supply, switch means in said circuit, a phototube having an anode, a light-sensitive cathode and multiplying electrodes, a fluorescent screen positioned adjacent said photo-tube, a trigger tube having an anode, a cathode and a control grid, an electrical circuit sensitive to iiring of said trigger tube for actuating said switch means to terminate said X-ray exposure consequent upon operation of said X-ray tube under normal conditions for a predetermined length of time, a second trigger tube having an anode, a cathode and a control grid, an electrical circuit coupling the second trigger tube to the photo-tube and including means for utilizing the photo-tube current to iire said second trigger tube consequent upon the impingement of an abnormal amount of radiation upon said iiuorescent screen during the X-ray exposure, and electro-responsive switch having energizing means in said coupling circuit responsive to the firing of said second trigger tubes for positioning the switch to terminate the operation of the photo-tube and the X-ray tube.

5. In combination, an X-ray tube, an electrical circuit connecting said tube to a source of current supply, switch means in said circuit, a phototube having an anode, a light-sensitive cathode and multiplying electrodes, an energizing circuit for conditioning said photo-tube for operation, a iiuorescent screen positioned adjacent said photo-tube, a trigger tube having an anode. a cathode and a control grid, an electrical circuit including a capacitor sensitive to firing of said trigger tube for actuating said switch means to disconnect said X-ray tube from its source consequent upon the charging of the capacitor under normal conditions of impingement of radiation upon said screen during operation of said X-ray tube, a second trigger tube having an anode, a cathode and a control grid, an electrical circuit coupling the second trigger tube to the photothe phototube current to fire said second trigger tube independently of the charge on said capacitor consequent upon the impingement of an abnormal amount of radiation upon said fluorescent screen during the X-ray exposure, and an electro-responsive switch having energizing means in said coupling circuit responsive to the iiring of said second trigger tube for positioning the switch to terminate the operation of the photo-tube and the X-ray tube.

6. In an X-ray exposure control circuit having a photo-tube responsive to X-ray radiations from an X-ray generator, a source of electrical energy for activating the X-ray generator, means for electrically conditioning the photo-tube to ac* tively respond to -Xray radiations by developing an electrical current therein proportional to the impinging quantity of X-ray radiation,

means for isolating the source from the X-ray generator consequent upon a predetermined X-ray exposure time interval, and electro re sponsive means for isolating the source from the X-ray generator in response to a predetermined maximum amount of X-ray radiation and including a control circuit having a voltage responsive trigger tube normally isolating said electroresponsive means fromthe photo-tube, a resistthe impinging quantity of X-ray radiations, circuit means including a switch for removably connecting the source to the X-ray generator, a control circuit having electrically responsive means for opening such switch consequent upon a predetermined quantity of electrical current flowing through the photo-tube, said control circuit including a Voltage responsive trigger tube normally isolating said electrically responsive means from the current in said electrical tube, storage means coupled between the photo-tube and the trigger tube to develop a voltage sufn cient to re the trigger tube and open the switch, a second trigger tube responsive to a higher voltage in said control circuit normally isolating said electrically responsive means from the current in said photo-tube, a resistance coupled between the photo-tube and said second trigger tube and adapted to develop the higher voltage to fire the second trigger tube and open the switch, and means responsive to the ring of said second trigger tube for removing said electrical conditioning means from the photo-tube.

References Cited in the file Of this patent UNITED STATES PATENTS Number Name Date 2,401,289 Morgan et al. May 28, 1946 2,412,423 Rajchman et al. Dec. l0, 1946 2,442,985 Rajchman June 8, 1948 2,454,169 Haynes Nov. 16, 194?, 2,541,187 Ball et al Feb. 13, 19.51 2,571,838 Connor et al Oct. 16, 1951 2,640,160 Collins May 26, 1953 i S v 

